Abstract
The Songnen Plain of the Northeast China is one of the three largest soda saline-alkali regions worldwide. To better understand soil alkalinization and salinization in this important agricultural region, it is vital to explore the distribution and variation of soil alkalinity and salinity in space and time. This study examined soil properties and identified the variables to extract soil alkalinity and salinity via physico-chemical, statistical, spectral, and image analysis. The physico-chemical and statistical results suggested that alkaline soils, coming from the main solute Na2CO3 and NaHCO3 in parent rocks, characterized the study area. The pH and electric conductivity (EC ) were correlated with both narrow band and broad band reflectance. For soil pH, the sensitive bands were in short wavelength (VIS) and the band with the highest correlation was 475 nm (r = 0.84). For soil EC, the sensitive bands were also in VIS and the band with the highest correlation was 354 nm (r = 0.84). With the stepwise regression, it was found that the pH was sensitive to reflectance of OLI band 2 and band 6, while the EC was only sensitive to band 1. The R2Adj (0.73 and 0.72) and root mean square error (RMSE) (0.98 and 1.07 dS/m) indicated that, the two stepwise regression models could estimate soil alkalinity and salinity with a considerable accuracy. Spatial distributions of soil alkalinity and salinity were mapped from the OLI image with the RMSE of 1.01 and 0.64 dS/m, respectively. Soil alkalinity was related to salinity but most soils in the study area were non-saline soils. The area of alkaline soils was 44.46% of the basin. Highly alkaline soils were close to the Zhalong wetland and downstream of rivers, which could become a severe concern for crop productivity in this area.
Highlights
Soil alkalinization and salinization is the process of accumulation of free salts such as Na+, K+, Mg2+, and Clin subsoil and groundwater, which leads to soil degradation and hinders the growth of plants [1,2]
It is commonly accepted that soil with pH above 7.5 is considered alkaline [15,16]
The aim of this study is to investigate the potential of mapping soil alkalinity and salinity from the Landsat 8 OLI image assisted with soil physico-chemical measures and spectra in northern Songnen
Summary
Soil alkalinization and salinization is the process of accumulation of free salts such as Na+ , K+ , Mg2+ , and Clin subsoil and groundwater, which leads to soil degradation and hinders the growth of plants [1,2]. When Na+ is the main source of salinization, it is called sodification that is usually associated with increasing pH of soil. Development of salinity, alkalinity, and sodicity in soils decreases productivity in croplands and limits the diversity of grass species in pastureland [3]. The Songnen Plain in the Northeast China is one of the three largest soda saline-alkali regions in the world [4]. The saline-alkali soils in this region can be explained by soil parent material, terrain, and climate. The plain is surrounded by the Changbai, Xiaoxing’an, and Daxing’an
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